This invention relates to a novel and improved method for the preparation of shaped articles of a fluorinated elastomer or, in particular, of shaped articles of a fluorinated elastomer with high mechanical strengths and excellent heat-shrinkability.
Fluorinated elastomers are very valuable because of their high heat stability and resistance against chemicals and are widely employed in diversified industrial fields, such as, the manufacture of vehicles, aircraft, electric appliances and the like. Shaped articles from conventional fluorinated elastomers are, however, not always satisfactory in their mechanical strengths, especially, at elevated temperature. The mechanical strength, for example, the elongation, tends to decrease markedly after prolonged exposure to high temperatures. Therefore the temperature range in which they can be used is disadvantageously limited.
Methods for preparing polymer blends have been proposed to overcome the above disadvantages of the fluorinated elastomers. For example, a rubber blend is obtained by blending a fluorinated elastomer with various elastomeric polymers, such as, synthetic rubbers of acrylonitrile and butadiene and silicone rubbers with or without fluorine-substituted organic groups. Improvement of some of the properties of the fluorinated elastomers may be achieved by the method of rubber blending. However, satisfactory results with respect to the mechanical properties due to poor affinity between the fluorinated elastomer and the elastomeric polymers have not been obtained.
Alternatively, a resin blend of a fluorinated elastomer has been proposed by cold-milling with a poly(vinylidene fluoride) resin and other optional ingredients (see, for example, U.S. Pat. No. 3,769,371). The resin blend thus obtained is found somewhat superior compared to the abovementioned rubber blends in the stability of the mechanical properties, especially elongation, on prolonged exposure to an elevated temperature even though their electric properties and mechanical properties themselves are rather poor.
Further, heat-shrinkable shaped articles based on a fluorinated elastomer may be prepared by a method, in which the fluorinated elastomer is blended with various kinds of thermoplastic resins into a resin blend, and the resin blend is molded into a shaped article under stretching and cooled before the stretching force is released. The shaped articles made of such resin blends are inferior in heat stability and resistance against chemicals inherent in the fluorinated elastomer itself along with somewhat decreased mechanical strengths.
It may be added that heat-shrinkable articles having satisfactory properties have hitherto been thought not to be obtainable from a resin blend composed of a fluorinated elestomer and a poly(vinylidene fluoride) resin (see British Pat. No. 1,250,503).